The pineal gland (or pineal body as it is also known) appears repeatedly in the literature of the history of anatomy and medicine. In the second century AD, Galen, one of the founding fathers of modern anatomy and a massive influence on the development of medicine, saw it as a secreting gland, a view similar to the one held today. Later in the seventeenth century, Descartes challenged this view. He was a strong advocate of 'dualism', the notion that the body and the spirit were separate entities and proposed that the pineal was in fact the seat of the soul. Wharton, again in the seventeenth century, on a very different note, thought that it was in fact a valve between the third and fourth ventricles of the brain due to its position between these two fluid-filled chambers of the brain. At the very end of the seventeenth century, in 1695, Ridley, in his book Anatomy of the Brain, proposed that it was in fact a lymph gland, taking fluid from the lymph ducts surrounding the ventricles. This view persisted right up until the beginning of the twentieth century! The present view is that its function is not clear, perhaps less attractive an idea than the Romantic notions of Descartes et al., but one more in keeping with the evidence. Studies suggest it has some role in maintaining the body's internal clock. This is dealt with much more in the section 'What does it do?'.